Quantitative interpretation of the physical processes associated with the interannual variation of South Asian summer monsoon

The South Asian summer monsoon (SASM) is of considerable scientific and social importance to the densely populated South Asia. Existing literature signified that the interannual variation of the SASM can be reflected by multiple dynamical and radiative processes. However, quantifying their relative contributions remains inadequate, particular for the contribution of aerosol process. Here, the land-sea thermal contrast index (LSTCI) is employed to represent the large-scale thermal driving at the mid–upper troposphere associated with the SASM, which is defined as temperature difference between the southern Eurasia (SE) and the tropical Indian Ocean (TIO) at the mid–upper troposphere. Based on the coupled atmosphere-surface climate feedback-response analysis method, this study linearly decomposes the total temperature change associated with the LSTCI into several partial temperature changes associated with individual dynamical and radiative processes. Our result demonstrates that the LSTCI is mainly explained by the positive contributions of atmospheric dynamic (69%), water vapor (35%), and aerosol (11%) processes, which are partially offset by a negative contribution of cloud process (-18%). Surface dynamic process play a neglectable role in the LSTCI, because it exerts similar effects on the temperature anomalies over the SE and the TIO. Further analysis indicates that the total effect of aerosols is dominated by change in black carbon. As two important components, the temperature anomalies over the SE and the TIO separately account for about 55% and 45% to the LSTCI. Our finding provides a new insight onto quantitatively understanding the relevant processes involved in the SASM variation.